This invention relates generally to ionizing radiation imaging systems and, more particularly, to systems and methods for coupling a scintillator to a light imager.
In one common imaging system configuration, an X-ray source projects an X-ray beam that passes through the object being imaged, such as an aircraft engine component. The beam, after being attenuated by the object, impinges upon a detector having an array of detector elements. The intensity of the radiation beam received at the detector is dependent upon the attenuation of the X-ray beam by the object. Each detector element of the array produces a separate electrical signal that is representative of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce an image. Multiple images may be taken of the same object, for example, by a series of rotations, and the set of images processed to form a cross-sectional image of the X-ray attenuation of the object.
The radiation imaging system commonly comprises a light imager (e.g., a photosensor array) coupled to a scintillator. The photosensor array comprises a plurality of pixels, each having a photosensitive element, such as a photodiode, and pixels are configured into a matrix of rows and several columns, e.g., about one thousand rows and columns. The scintillator includes scintillation material positioned over the imager array. As described below, the scintillator may be integral with the imager array, for use in detecting low energy (radiation less than about 100 keV), or a separate plate located over the imager array when the device is used for detection of high energy radiation (radiation above about 100 keV). Contact pads are coupled to or formed on the imager array adjacent the periphery of the imager array and are associated with the respective rows and columns in the imager array. Particularly, the contact pads facilitate accessing information from each row and column of the photosensing element array by enabling electrical contact to external circuitry.
The above-described system sometimes is referred to as a computed tomography (CT) system. Although the present invention is sometimes described in the context of CT systems, the present invention is not limited to use in connection with CT systems and can be utilized with other radiation based imaging systems, such as radiographic X-ray systems.
During scanning, X-rays are emitted from the X-ray source in the direction of the detector, and each X-ray, which interacts with the scintillator, is converted into visible photons in accordance with the scintillator gain. For example, a scintillator having a gain of 1000 converts each X-ray from the X-ray source, on average, into 1000 photons. These photons are detected by photosensors that develop an electrical signal (e.g., charge accumulation on a photodiode) corresponding to the detected photons. This accumulated electrical signal on photosensors in the array is accessed via the contact pads and used by readout electronics to provide an estimate of the location of the ray event. Further digital processing is used to integrate the signal from all elements of the photosensor array, and from multiple images if more than one scan is taken, and to form the acquired image.
For low energy radiation, a scintillator deposited directly on the light imager may be used. Due to the practical thickness limitations of deposition, on the order of 1 mm, for high energy radiation (i.e., radiation above about 100 keV), the scintillator typically is a separate plate coupled to the imaging plate so that a surface of the scintillating plate is adjacent the imaging plate.
In small ionizing radiation imagers, utilizing a separate scintillator plate generally provides satisfactory results. However, use of a separate scintillator plate may result in degradation of image quality in the larger two-dimensional, or area, ionizing radiation imagers. For example, directly coupling a large, e.g., greater than 100-cm2 scintillator to a large, e.g., greater than 100-cm2 scintillator, a light imager suffers from response variation due to a varying air gap between the scintillator and the light imager. In addition, imaging systems utilizing a scintillator coupled to a light imager may be susceptible to oversaturation. For example, if an X-ray source emits approximately 300,000 rays to generate an image, the scintillator produces approximately 300,000,000 photons. This photon level may exceed the capacity of the light imager system circuitry depending on the read time and charge capacity of the photosensing element (which in turn depends on the common voltage bias and area of each element). A typical common bias is about 10 volts (V) and a typical element size is about 0.01 to 1.0 mm2. One way to avoid oversaturation is to perform multiple readouts. Performing multiple readouts, however, may result in excessively long total readout time for the part being imaged and increases noise for a single image. Until now, to prevent oversaturation, either the X-ray flux or the number of photons generated in the scintillator for each incident ray is reduced. Reducing the flux, however, degrades the system signal-to-noise ratio, which is undesirable.
It would be desirable to provide improved optical quality in high energy imaging systems with large imagers. The optical quality can be assessed using two quantitative measurements. The first quantitative measurement is the modulation transfer function (MTF), which represents a measure of the light spread. The second quantitative measurement is the detector quantum efficiency, which includes both the MTF and the noise terms; therefore, the detector quantum efficiency is a reflection of signal-to-noise ratio of the image detectability. It also would be desirable to reduce the likelihood of system oversaturation without significantly reducing signal-to-noise ratio or increasing readout time. It further would be desirable to provide a simple method for fabricating such an improved detector.
A radiation imaging system is provided comprising a scintillator, an imager array, and a lamination layer. The lamination layer bonds and optically couples the scintillator to the imager array. The lamination layer is comprised of a lamination material that is substantially free from void spaces.
A method for fabricating a radiation imaging system is provided comprising the steps of disposing a lamination layer between a light imager and a scintillator to form a subassembly. The light imager comprises an imager array, an imaging plate surface and a plurality of contact pads. Additional steps include subjecting the subassembly to a vacuum; heating the subassembly to a bonding temperature, exerting a bonding force on the subassembly, maintaining the vacuum, the bonding temperature and the bonding force until the light imager is bonded to the scintillator and the lamination layer is comprised of a lamination material that is substantially free from void spaces.